Download TND6197 - Stepping Motors and Stepping Motor Control System

TND6197/D
Stepping Motors
and Stepping Motor
Control System
Abstract
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This tutorial covers the basic principles of stepping
motors and stepping motor control systems, including both
the physics of steppers, the electronics of the basic control
systems, and software architectures appropriate for motor
control.
TECHNICAL NOTE
Introduction
3. Brushless DC Motors − These motors require
servo control with an encoder for feedback as well
as external electronic commutation. They have no
brushes however, and they provide high torque
over a large speed range. Compared to DC Brush
motors, brushless DC motors today offer the
‘go to’ choice in servo control applications
because their historically higher costs have come
way down.
Stepping motors can be viewed as electric motors without
commutation. Typically, all windings in the motor are part
of the stator, and the rotor is either a permanent magnet or,
in the case of variable reluctance motors, a toothed block of
some magnetically soft material. All of the commutation
must be handled externally by the motor controller, and
typically, the motors and controllers are designed so that the
motor may be held in any fixed position as well as being
rotated one way or the other. Most steppers, as they are also
known, can be stepped at audio frequencies, allowing them
to spin quite quickly, and with an appropriate controller, they
may be started and stopped “on a dime” at controlled
orientations.
Stepper motors are predominantly used for positioning
applications, but they aren’t the only game in town for that
task. So let’s start by pulling the camera way back and
checking your overall choices.
Below is basic information on common motor choices for
positioning motor applications. Note that there are a few
other possible choices, such as AC Induction motors or
Piezo motors, but these three listed motor types represent the
huge majority of applications in use today for general
purpose motion control.
1. Stepper Motors − These motors are
self-positioning, and therefore simple to use. They
do not require an encoder to maintain their
position nor do they require a servo control loop.
Their main drawbacks are vibration and noise,
and limited speed range. Like brushless DC
motors they must be “commutated” externally
using a multi-phase drive.
2. DC Brush Motors − These motors require a
position encoder for feedback and are stabilized
using a PID (Proportional, Integral and Derivative)
controller or other position loop controller. These
motors do not require external phasing − give them
some current and off they go. However, the
mechanical brushes inside the motor which
accomplish commutation may eventually wear out
and fail.
© Semiconductor Components Industries, LLC, 2015
December, 2015 − Rev. 1
For some applications, there is a choice between using
servomotors and stepping motors. Both types of motors
offer similar opportunities for precise positioning, but they
differ in a number of ways. Servomotors require analog
feedback control systems of some type. Typically, this
involves a potentiometer to provide feedback about the rotor
position, and some mix of circuitry to drive a current through
the motor inversely proportional to the difference between
the desired position and the current position.
In making a choice between steppers and servos, a number
of issues must be considered; which of these will matter
depends on the application. For example, the repeatability of
positioning done with a stepping motor depends on the
geometry of the motor rotor, while the repeatability of
positioning done with a servomotor generally depends on
the stability of the potentiometer and other analog
components in the feedback circuit.
Stepping motors can be used in simple open-loop control
systems; these are generally adequate for systems that
operate at low accelerations with static loads, but closed
loop control may be essential for high accelerations,
particularly if they involve variable loads. If a stepper in an
open-loop control system is over-torque, all knowledge of
rotor position is lost and the system must be reinitialized;
servomotors are not subject to this problem.
Introduction to Stepper Motor Types
Stepper motors come in two varieties, permanent magnet
and variable reluctance (there are also hybrid motors, which
are indistinguishable from permanent magnet motors from
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the controller’s point of view). Lacking a label on the motor,
you can generally tell the two apart by feel when no power
is applied. Permanent magnet motors tend to “cog” as you
twist the rotor with your fingers, while variable reluctance
motors almost spin freely (although they may cog slightly
because of residual magnetization in the rotor).
You can also distinguish between the two varieties with an
ohmmeter. Variable reluctance motors usually have three
(sometimes four) windings, with a common return, while
permanent magnet motors usually have two independent
windings, with or without center taps. Center-tapped
windings are used in unipolar permanent magnet motors.
Stepping motors come in a wide range of angular
resolution. The coarsest motors typically turn 90 degrees per
step, while high resolution permanent magnet motors are
commonly able to handle 1.8 or even 0.72 degrees per step.
With an appropriate controller, most permanent magnet and
hybrid motors can be run in half-steps, and some controllers
can handle smaller fractional steps or micro steps.
For both permanent magnet and variable reluctance
stepping motors, if just one winding of the motor is
energized, the rotor (under no load) will snap to a fixed angle
and then hold that angle until the torque exceeds the holding
torque of the motor, at which point, the rotor will turn, trying
to hold at each successive equilibrium point.
Inherent detent torque
Bi-directional operation
Can be stalled without motor damage
No brushes for longer trouble free life
Precision ball bearings (depending on brand/type)
Drawbacks of Stepper Motors
• Resonances can occur if not properly controlled
• Not easy to operate at extremely high speeds
• If over torqued, all knowledge of position is lost and
system must be reinitialized
• Produces much less torque, for a given size than the
equivalent DC/AC motor
Winding Connections
Stepper motors are produced in a number of different lead
configurations. The most popular are:
• 4 Lead − Bipolar
• 5 Lead − Unipolar
• 6 Lead − Unipolar, Bipolar (series connected)
• 8 Lead – Unipolar, Bipolar (series connected) &
Bipolar (parallel connected)
Hybrid Stepper
This version of a Stepper Motor is a clever combination
of the variable reluctance and permanent-magnet types.
Stepper Modes
The stepper motor can be driven in a number of different
sequences. The most common of these are:
• Wave Drive
In this mode only one phase is energized at any given
time. For unipolar motors this means only 25% of the
available windings are Utilized or 50% utilization for
bipolar motors.
• Full Step Drive
In this mode two phases are energized at any given
time. For Unipolar motors this means 50% of the
available windings are utilized, or 100% utilization for
bipolar motors.
• Half Step Drive
In this mode the sequences of the wave and full-step
drives are interleaved to enable the rotor to be aligned
in half steps. For unipolar Motors this means 37.5% of
the available windings are utilized (on average) or 75%
utilization for bipolar motors
Stepper Motor Fundamentals
Applications
Variable Reluctance
This type of motor does not use a permanent magnet. As
a result, the rotor can move without constraint or ‘detent’
torque. This type of construction is the least common and is
generally used in applications that do not require a high
degree of torque, such as the positioning of a micro slide.
Permanent Magnet
Also referred to as a ‘canstack’ or ‘tincan’ motor, this
device has a permanent magnet rotor. It is a relatively low
speed, low torque device with large step angles of either 45
or 90 degrees. The simple construction enables these motors
to be produced at low cost, making them the ideal choice for
low power applications.
• Industrial Machines
Features of Stepper Motors
• Rotation angle is proportional to number of input pulses
• Rotational speed is proportional to the frequency of
input pulses
• Open loop system with no position feedback required
• Excellent response to acceleration, deceleration and
step commands
• Non-cumulative positioning error (±5% of step angle)
• Excellent low speed and high torque characteristics
without need for gear reduction
• Holding torque when energized
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Stepper motors are used in automotive gauges and
machine tooling automated production equipments.
Robotics in MFG, Inspection and Process Flow.
Security
New surveillance products for the security industry.
Including Security Cameras PAN/ZOOM/TILT.
Medical
Stepper motors are used inside medical scanners,
samplers, and also found inside digital dental
photography, fluid pumps, respirators and blood
analysis machinery.
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and Stray Currents needs to be accounted for and driver has
to be efficient, small and easy to integrate into an Analog
Motor World.
With emphasis on cost, accuracy and ease of use the World
has turned to a Stepper Motor Applications. As stated in
earlier paragraphs, In Theory, a Stepper motor is a marvel in
simplicity. It has no brushes or contacts. Basically it’s
a synchronous motor with the magnetic field electronically
switched to rotate the armature magnet. It converts digital
pulses into mechanical shaft rotation so in other words it’s
a “Digital Motor”.
Stepper motors in cameras for automatic digital camera
focus and zoom functions.
And also have business machines applications,
computer peripherals applications.
ON Semiconductor Stepper Motor Driver Solutions
With increased focus on Security, Medical and industrial
Applications, as well as reducing cost of System Level
design and assembly, every Manufacturer turned to higher
levels of integrations as well as the cost down overall. High
level of Integration in the Digital World is nothing new,
every 18 month the speed doubles and size of Integrated
Circuits reduced to close to 50%. In Power Analog World
that Motors usually located in it’s not as easy as it seems.
Motor Horse Power, Heat Dissipation during Motor Drive
V+
Controlling the Current in Stepper Motor
Controlling the current in the 2 motor coils using
H-bridges − Voltage mode.
V+
V+
V+
PWM
A
A
B
A
S N
A
PWM
N
B
S
PWM
PWM
B
B
iA
iA
5
4
t
iB
t
iB
t
t
Figure 1. Voltage Mode
Figure 2. PWM Mode − Microstepping
when the MAX delivered torque equals the Load. Key Point
– MAX Torque is delivered at the edge of the stable region
and if not taken care of in the Driver Logic, will risk to stall
the Motor.
The Stepper Motor Driver has to Depend on the
Following Key Factors
Torque/Torque Efficiency:
Torque is the force to rotate an object around an axis. To
generate Torque Two Elements are required – Rotor Filed
generated by Permanent Magnet and Stator Field generated
by Stator Current. Torque will be MAX when the Fields have
Opposite Direction − at q = −p/2 and q = p/2. Torque will
increase by increasing the coil current and MAX Efficiency
B
Load Angle:
Load angle d is the angle between stator and rotor field. It
is the equilibrium “p” between generated torque and load.
With increasing load the load angle d will increase also.
Fm
eemk
wt
Fm
N
iS
d
eemk
iS
wt
A
A
p/2
q
S
d
B
Figure 3. Load Angle
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Current Control:
By using Pulse Width Modulation (PWM) and switching
H-Bridge the Votlage4 will be chopped across the coil,
winding of the Motor. When The Voltage develops across
the Coil the Current will increase. When the Coil is shorted,
the current will decrease, also called “DECAY”.
I
V+
PWM
t
I
PWM
PWM
Fixed to
GND
t
PWM
t
Figure 4. Current Control
The Key Issues Facing Stepper Motor Drivers and
Applications
Full Design of a Pan/Tilt Sec Camera in weeks and not
months. With LV8714A both Motors can be driver
simultaneously due to Reference Voltage Input avail for
each H-Bridge. With wide Input Voltage range of 4–16.5 V
it is perfect for any of the following camera applications:
• PoE Sec Camera
• PoE Point of Sales Terminal
• Document Scanners
• Assembly line Quality Control
• Flatbed scanner and Multifunction Printers
Security Cameras
With Security becoming Number One concern World
Wide the use of IP Security Cameras has quadrupled over the
last five years. With Digital Cameras and taking the
forefront of technology the biggest drawback is increased
use of low cost CMOS and CCD Image sensors. The Video
quality highly depends on the Sensor Resolution and
“Digital Artifacts”, Video Distortion, can be seen clearly
when Camera Pans or Tilts. Originally this issue was taken
care with expensive Analog Motor Drivers with high
resolution Servo Feedback based on the Shaft encoder
quality. Use of such Drivers was very expensive and
different approach had to be introduced for increase Video
Quality as well as reduced cost and complexity.
Multitude of Safety Factors is built in. Such as:
1. Single Supply Voltage for more reduced cost.
2. High Efficiency and lower Power consumption
due to Low Rds(on) internal FET’s.
3. Internal Current Sense resistors for reduced
components count and further reduction in Power
Dissipation. Bringing Power Consumption down
by 35% over the competition.
4. Complete Safety Design with Over-current and
Thermal Shutdown without use of external
components.
Introduce ON Semiconductor LV8714A
(Highly Integrated, Dual Channel, PWM Constant,
Current Control Stepper Motor Driver)
With Full Quad H-Bridge Power Switches housed
internally with Driver, PWM Logic, LV8714A can driver
Two Stepper Motors or Four DC Brushed Motors, if such
need arises. With LV8714 any Designer can go from Zero to
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Figure 5. Schematic
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−
+
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Current
Detect
AMP
RCS4
VREF4
1/3
Output Pre-Stage
RCS3
VREF3
IN4
Output Control Logic
ENA4
Current
Detect
AMP
IN3
OSC
TSD
LVS
OCP
+
−
OCP
LVS
TSD
OSC
Output Pre-Stage
ENA3
1/3
+
−
Current
Detect
AMP
RCS2
VREF2
1/3
OCP
LVS
TSD
Oscillator
Output Pre-Stage
RCS1
VREF1
IN2
ENA2
IN1
ENA1
OSC
TSD
LVS
OCP
VM −3.3 V Regulator
3.3 V Regulator
Start-Up Circuit
PGND3
PGND4
OUT4B
OUT4A
OUT3B
OUT3A
PGND1
VM3, VM4
PGND2
OUT2B
OUT2A
OUT1B
OUT1A
VM1, VM2
+
GND
VREG3
PS
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Output Pre-Stage
Current
Detect
AMP
Output Control Logic
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